This invention relates to solid polyurethane elastomers which exhibit reduced shrink and to a process for the production of these elastomers. These elastomers comprise the reaction product of a polyisocyanate component or prepolymer, with an isocyanate-reactive component comprising a high molecular weight component having an EO cap of less than about 17% by wt., a chain extender and optionally, a crosslinker, in the presence of at least one catalysts. These solid elastomers are produced via the reaction injection molding process in a closed mold, at an isocyanate index of about 70 to about 130.
The production of polyurethane moldings via the reaction injection molding (i.e. RIM) technique is well known and described in, for example, U.S. Pat. No. 4,218,543. The RIM process involves a technique of filling the mold by which highly reactive, liquid starting components are injected into the mold within a very short time by means of a high output, high pressure dosing apparatus after they have been mixed in so-called “positively controlled mixing heads”.
In the production of polyurethane moldings via the RIM process, the reaction mixture generally comprises an A-side based on polyisocyanates and a B-side based on organic compounds containing isocyanate-reactive hydrogen atoms, in addition to suitable chain extenders, catalysts, blowing agents, and other additives. The polyisocyanates which are typically used in a commercial RIM process are the aromatic isocyanates such as, for example, diphenyl methane-4,4′-diisocyanate (i.e. MDI).
A RIM process and polyurethane integral skin foams produced by that process are disclosed in U.S. Pat. No. 4,243,760. These integral skin foams comprise the reaction product of an aromatic isocyanate, a high molecular weight polyether triol, and a chain extender. The high molecular weight polyether is characterized as having an internal mixed propylene oxide/ethylene oxide segment and a sufficient EO cap to result in a primary hydroxyl group content of >50%, and usually >90%. These integral skin foams have a compact skin or surface and a cellular foam core, and require a blowing agent.
RIM elastomers that are dimensionally stable at high temperatures and a process for their production are described in U.S. Pat. No. 4,297,444. These comprise the reaction product of a polyether polyol, a relatively low molecular weight compound containing at least two active hydrogen groups, and a polyisocyanate. The polyisocyanate and at least about 30% of the polyether polyol are pre-reacted to form a prepolymer, and the balance of polyether polyol (if any) is mixed with the low molecular weight compound, which is then added to the prepolymer and reacted. The polyisocyanate component may be a blend of a prepolymer and a polyisocyanate. It is also disclosed in the '444 patent that shrinkage factors are similar after 250° F. postcure and 325° F. postcure when more polyol is introduced on the A-side in the form of a quasi-prepolymer.
U.S. Pat. No. 4,440,705 describes a process for preparing cellular and non-cellular polyurethane elastomers, preferably by a RIM process. The process comprises reacting an organic isocyanate, with a polyether polyol, chain extender and/or crosslinking agent in the presence of catalysts, blowing agents, and various additives. The polyether polyols have terminal hydroxyethylene groups with ethoxylation degrees of 1 to 12, preferably 2 to 8, and a high content of primary hydroxyl groups. All of the examples are cellular elastomers prepared with a blowing agent (dichlorodifluoromethane). The resultant integral skin foams exhibited shorter demold times, and good dimensional stability (i.e. no shrinking or swelling), along with good elongation and bending stability.
U.S. Pat. No. 5,238,973 discloses a process for preparing elastic moldings which are optionally cellular by the RIM process in a closed mold. This process comprises reacting a prepolymer which comprises the reaction product of an MDI-based isocyanate component with a polyether polyol (MW=1000 to 6000 and OH functionality of no more than 2.4), at an equivalent ratio of at least 3.2:1 to form a semi-prepolymer, and reacting the prepolymer via the RIM process with B) which consists essentially of up to 10% of the polyol b) and c) an aromatic diamine having an alkyl substituent in at least one ortho-position to the amino groups, and optionally d) low molecular weight aliphatic polyols or aliphatic aminopolyethers. At least 90% by weight of the high molecular weight polyol is used to make the prepolymer in the first step.
A RIM process comprising reacting a NCO-terminated prepolymer and one or more NCO-reactive components which contain inorganic fillers is disclosed in U.S. Pat. No. 5,504,179. A compound which contains an acidic group is typically included in the NCO-reactive component. Suitable prepolymers include those of any isocyanate with a polyol having a MW of 1500 to 12,000 and OH functionality of at least 2. Suitable isocyanate-reactive components comprise (1) aromatic diamines having at least one alkyl substituent in a position ortho to the amino groups (e.g. DETDA) and (2) aliphatic reactive components having OH or NH functionalities of 2 to 6 and MWs of 230 to 12,000, and being a polyether or polyester which contains OH and/or primary NH groups in an amount of from 5 to 20 equivalent %. It is disclosed that surprisingly the invention shows the effect of a reduced shrinkage due to the acidic component. The shrinkage with the acidic component in the present invention is essentially the same as seen in a single step process. The use of fillers in these elastomers is also known to reduce shrinkage.
High performance RIM polyurethane(urea) elastomers are described in U.S. Pat. No. 6,765,080. These comprise the reaction product of (A) an allophanate-modified diphenylmethane diisocyanate prepolymer having an NCO group content of 5 to 20%, and (B) an isocyanate-reactive component comprising (1) a high molecular weight ATPE having a functionality of 2 to 5, (2) an aromatic diamine chain extender having a molecular weight of 100 to 500, (3) optionally, chain extenders and/or crosslinkers having a molecular weight of 200 to 600, a functionality of 1.5 to 6, and are either aliphatic amine terminated polyether polyols or aliphatic hydroxyl terminated polyether polyols.
Advantages of the present invention include the ability to control and/or reduce shrinkage of solid polyurethane elastomers without the addition or inclusion of a special additive such as inorganic fillers as in U.S. Pat. No. 5,238,973 and/or acidic additives as described in U.S. Pat. No. 5,504,179.